Laser-induced desorption of NO from NiO(100): Ab initio calculations of potential surfaces for intermediate excited states

Abstract

In order to interpret the experimental results of the state resolved UV-laser-induced desorption of NO from NiO͑100͒ ͑rotational and vibrational populations, velocity distributions of the desorbing NO molecules, etc.͒, we have performed ab initio complete active space self-consistent field ͑CASSCF͒ and configuration interaction ͑CI͒ calculations for the interaction potential between NO and the NiO͑100͒ surface in the electronic ground state and for those excited states which are involved in the desorption process. The NiO͑100͒-NO distance and the tilt angle between the NO axis and the surface normal have been varied. A cluster model containing a NiO 5 8Ϫ-cluster embedded in a Madelung potential has been used for representing the NiO͑100͒ surface. The excited states which are important for the desorption process, are charge transfer states of the substrate-adsorbate system, in which one electron is transferred from the surface into the NO-2-orbital. The potential curves of these excited charge transfer states show deep minima ͑4 eV-5 eV͒ at surface/NO distances which are smaller than that in the ground state. The angular dependence of these potentials behaves similar as in the ground state. A semiempirical correction to the calculated excitation energies has been added which makes use of the bulk polarization of NiO. With this correction the charge transfer states are considerably stabilized. The lowest excitation energy amounts to about 4 eV which is in reasonable agreement with the onset of the laser desorption observed experimentally at about 3.5 eV. The density of the NO Ϫ-like states is rather high, so that probably several excited states are involved in the desorption process. The potential energy curves for all of these states are quite similar, but the transitions from the ground state into different excited charge transfer states show strongly differing oscillator strengths, which are also strongly dependent on the surface/NO distance. This fact is important for the dynamics of the deexcitation process in the sense of a selection criterion for the states involved. The magnitude of the oscillator strengths is large in comparison with the excitation of NO in the gas phase, which might be an indication for the possibility of optical excitation processes. One dimensional wave packet calculations on two potential energy curves using fixed lifetimes for the excited state in each calculation have been performed and enable us to estimate the mean lifetime of the excited state to be 15 fsрр25 fs. This implies that the dynamics of the system is dominated by the attractive part of the excited state potential.

Chromophore-specific theoretical spectroscopy: From subsystem density functional theory to mode-specific vibrational spectroscopy

“…[150] for cluster-in-cluster embedding methods or ''dipped adcluster'' embedding). However, this discrete representation of the environmental potential sometimes causes a distortion in the electron density distribution of the embedded cluster [151].…”

“…In the last line we inserted the expression for δρ(ω, r) according to Eqs. (151) and (152) obtained within the subsystem-TDDFT formalism and explicitly stated its frequency dependence.…”

Quantum Dynamical Approach to Ultrafast Molecular Desorption from Surfaces

“…[243][244][245][246][247][248] The situation is even more difficult for excited states. Configuration interaction (CI) and multiconfiguration selfconsistent field (MCSCF) techniques 239,[249][250][251][252][253][254] can be used for adsorbates on ionic and semiconductor surfaces. Meanwhile, also TD-DFT methods 255 are being used for the same purpose.…”

“…Meanwhile, also TD-DFT methods 255 are being used for the same purpose. 254 Even for oxide surfaces, the density of excited states can be very large, 249 and a multitude of different types of excitations may be located in the relevant energy region, e.g., metal-to-ligand, metal-to-oxygen, ligand-to-ligand excitations, etc. Furthermore, the energetic location of excited states is sensitive to details of the embedding scheme.…”

Photon‐induced field desorption of NO from Rh

“…In general, the high efficiency of photon-stimulated desorption of NO from oxides has been explained by a local charge transfer from the oxidic binding state to the adsorbed NO. 24 We therefore simulated the 'conditioning effect' by pre-adsorbing oxygen on the Rh tip, thereby producing observable enhancement of the PSFD yield. The surface composition of such a tip after preparation showed the same features as in Fig.…”